CZ282537B6 - Process for producing electrically conducting connection - Google Patents

Abstract

A method of forming an electrically conductive joint between a carbon body and a metal body comprises the steps of: i) providing a porous metal coating on a face of the carbon body; and ii) bonding said face to the metal body with an adhesive which penetrates the porosity of the metal coating and penetrates the porosity of the carbon body. By this method the porous metal coating provides an electrically conductive path, and the adhesive provides a bond, between the carbon body and the metal body. The metal body may also be provided with a porous metal coating.

Description

Method for producing an electrically conductive connection

Technical field

The invention relates to a method for manufacturing an electrically conductive connection between a carbon body having a carbon contact surface and a metal body having a metal contact surface. The invention is particularly, but not exclusively, applicable for attaching carbon blocks to aluminum carriers in the current collectors. Current collectors are used, inter alia, for electric trams, railway locomotives and railcars.

BACKGROUND OF THE INVENTION

The current collectors consist of a metal support (usually aluminum) to which a carbon block is attached. The current collector is mounted on the vertically moving frame so that the carbon block can be held in contact with an overhead guide. The overhead line current passes through the carbon block, the carrier and from there the cables to the vehicle engine.

There are many ways of attaching carbon to its metallic support. Soldering is not very practical because conventional solders do not wet carbon or aluminum and special solders and / or carbon or aluminum pretreatments are required, which is very difficult.

Synthetic resins (for example modified phenolic and epoxy resins, etc.) may form good bonds, but are non-conductive.

At present, the conductivity in such systems is achieved by filling the resin with conductive metal particles. This creates a high resistance bond and it is common to coat the copper before bonding with copper as described in Austrian Patent Specification No. 322629.

Furthermore, by creating metal strips, braids, grids, etc. at the joint between the carrier and the carbon block prior to bonding. This creates thick joints that are weak (usually 0.02-0.1 mm thick is considered optimal joint thickness, joints thicker or thinner usually have less shear strength J. Shields, 'Adhesives Handbook' Third Edition 1984 Butterworth) . The carbon is usually coated for the same reason as in a) above. The braid or grid can be inserted into the groove in carbon, but this limits the electrical contact area.

Copper coating, as mentioned in a) and b) above, is not entirely successful because the copper / carbon bond is weak and it is therefore necessary to make the coating very thick (up to 0.5 mm) and, for good strength, to stretch over the carbon sides and fit into the grooves. Such a thick coating is very expensive. GB 2202999 discloses electrical contact with contact surfaces formed by an electrically conductive adhesive-filled material such that contacting such electrical contact with another similar electrical contact produces no initial adhesion, but creates a permanent adhesive bond over time. This document does not deal with the problem of attaching carbon to metal.

It is an object of the present invention to provide a method for producing an electrically conductive connection between carbon and metal. It is an object of the present invention to provide articles comprising a joint formed by this method.

SUMMARY OF THE INVENTION

The present invention provides a method for producing an electrically conductive connection between a carbon body having a carbon contact surface and a metal body having a metal contact surface, which consists of:

From the steps of forming a porous metal coating on at least the carbon interface surface, and bonding the interface surfaces with an adhesive penetrating the pores of the metal coating and penetrating the pores of the carbon interface surface.

In the manufacture of joints using this method, good electrical conductivity is achieved through intimate contact between the porous coating and carbon, and the roughness and porosity of the porous coating ensures good electrical contact with the metal body. When both metal and carbon are coated, close direct contact between the porous coatings improves electrical conductivity, by their roughness and porosity, and interconnected at a plurality of points.

The porous metal coating may be applied, for example, by flame spraying or by arc spraying of copper or zinc, or by another suitable method.

The joint is thin because there are no metal particles or braid or lattice to hold the surfaces apart and the porosity of the metal coating allows the resin to penetrate the lower carbon or metal, slightly penetrating the carbon surface that is porous and thus providing a good joint without regardless of the bond between the carbon or metal and the metal coating.

The bonding resin must be sufficiently fluid at some stage to impregnate the porous metal coatings to achieve bonding, and therefore the original resin could be thin or thin during the curing heat treatment, or be composed of components on site.

The adhesive may be, for example, any suitable resin such as phenolic or epoxy. The resin may be cold or hot cure and may be in sheet form, if preferred.

To form the current collector, the coated carbon body and the resin-coated aluminum support are clamped together and released only after the resin has cured.

A detailed description of the invention with reference to the accompanying drawings will be given below.

List of drawings:

Giant. 1 is a cross-sectional view of the carbon / aluminum header body used for testing;

Giant. 2 is a schematic representation of the instruments used to measure resistance;

Giant. 3 is a schematic representation of apparatus used for shear strength testing.

Examples:

In FIG. 1, the test portion consists of a carbon body 1 and an aluminum body 2 extruded with a thickness of 2 mm. The dimensions are given in Table 1.

Table 1 a 35 mm b 20 mm c 25 mm d 21 mm

-2GB 282537 B6

The carbon surface 3 and the aluminum surface 4 to be bonded are sanded (44/60 silicon carbide) for roughening. Copper is applied to the carbon surface 3 and aluminum surface 4 using an oxygen / propane wire spray gun (Metallisation Co). The coating thickness is usually about 100 μιη. The upper thickness limit for this metal deposition method is approximately 300 μηι. Greater thickness and internal stress can cause metal to peel off from carbon. In addition, a greater thickness makes the resin difficult to penetrate.

The copper coating must be permeable to the resin during curing and this property is dependent on the porosity / permeability of the copper, the copper thickness and the viscosity of the resin. Different flame guns and other sputtering techniques of copper or other metal will provide different permeability and residual stress and hence different limit thickness. The resin type and curing conditions will control the viscosity of the resin. Another reason for using a thin metal coating is of course cost.

The minimum coating thickness is given by the need to achieve good electrical conductivity. 10 pm is probably the lower limit for small flat parts in good mechanical contact. For larger or curved parts where good mechanical contact (such as 1 meter long curved pantographs) is not so easy to achieve, thicker coatings of 25 - 50 µm or more are preferred.

The resin is applied to both the sandblasted surfaces, the carbon surface 3 and the aluminum surface 4, well spread over the entire surfaces to be joined. A suitable resin is Ciba MY750 with HT972 hardener mixed in a ratio of MY750 - 100 parts by weight, HT972 - 30 parts by weight. The mixture is heated to dissolve HT972 (solid) in MY750 (viscous liquid) and make the resin generally thinner and easier to use. The density of the resin used is the basis for experimentation and testing and depends on the porosity of the carbon body 1 used, the metal coating used and the cure rate of the resin. If the resin is too thin at any stage, a disproportionate amount of resin is sucked into the porous carbon body 1, leaving little for the joint. Too dense resin does not penetrate porous metal coatings and does not bond with carbon or aluminum. The dense cold-hardening resin can be improved by heating, which not only dilutes the resin but also increases its curing speed. There is a certain thinning / hardening balance between the heating rate and the curing temperature, as mentioned above, which can only be determined experimentally on a case-by-case basis.

As noted above, if the resin becomes too thin at any stage, this may lead to inadequate suction into the porous carbon body 1 and thus insufficient amount of resin is present to form the bond. This can be counteracted by thickening the resin with fine fillers such as silica powder, chalk powder, slate powder, sand powder or mica powder.

The parts to be joined are clamped together by a spring G clamp at approximately 10 cm intervals. Each spring exerts a force of about 20 kg (50 lb). Excess adhesive that is pushed out is wiped off. The resin is cured by placing in an oven at 150 ° C for 1.5 hours. The parts to be joined are cooled before the clamps are removed.

Resins other than epoxy resins are suitable, such as phenolic resins, polyester resins, melamins and silicones. Thermoplastics are less suitable due to the loss of properties at high temperatures but cannot be completely excluded.

It has been found that it is most advantageous to coat all surfaces to ensure maximum bonding area, but this is not necessary.

-3 CZ 282537 B6

For long parts, such as 1 m, clamping can be very important. If the clamping is too tight, the thermal expansion of carbon and aluminum can cause carbon breakage as the temperature rises. If the grip is too loose, there is a disproportionately high resistance to cooling after curing because the gap between carbon and aluminum is too wide. The mean pressure that will allow some carbon and aluminum relative displacement during heating but keep the gap narrow enough to provide a low resistance must be found by experiment.

For the production of long parts on a larger scale, a splitter can be used to avoid this staple problem. The method of use of such a press is as follows.

The parts to be bonded are inserted into the press after application of the bonding resin and are gently compressed together so as to allow carbon and aluminum movement due to relative thermal expansion.

The components are then heated to a medium temperature of, for example, 90 ° C so that most thermal expansion can freely apply, but the resin does not cure to a large extent. A higher pressure is then exerted in the press to achieve closer contact between carbon and aluminum and to prevent further relative displacement. Finally, the temperature is sharply raised to cure the resin (e.g., 120-130 ° C). This prevents carbon cracking by excessive thermal expansion and low resistance joints can be formed.

Although it has been found that for the MY750 / HT972 mixture described above, 150 ° C provides the best strength for 1 hour, other resin cure cycles are possible. Although this is not a necessary step, when using any resin, the curing cycle should be such that the resin is thin enough at some point to penetrate the copper - but the initial resin may already be thin enough.

Table 2 provides comparative information on several surface and resin treatment experiments, gives the electrical resistance between the top surface of the carbon body 1 and the aluminum body 2 for the 150 mm long test piece, and also shows the shear strength of the 30 mm test piece. Details of resistance and shear strength measurements are given below.

Examples of other resin systems, non-copper metal and other metal spraying methods are also shown in Table 2.

-4GB 282537 B6

Table 2.

exp. C. preprocessing resin the resistance in mQ between the upper surface of carbon and aluminum shear strength length 30 mm, kg, at ambient temperature shear strength length 30 mm, kg, at 160 ° C of aluminum carbon 51 70 sandblasting & spraying Cu 100 pm sandblasting & spraying Cu 100prn MY750 / HT972 cure at 150 ° C / 1 h 0.75 0.74 1635 1792 522 240.5 80 only sandblasting II II 0.87 540 118 79 sandblasting & spraying Cu 100 pm Cu plech 100 pm, based II after curing, Cu sheet and carbon were not joined - 26 only sandblasting only sandblasting Ell *, cure. Cold 41.7 - - 28 sandblasting & spraying Cu 100 pm 11 11 2.7 - - 24 only sandblasting Sandblasting & Spraying II 0.97 - - 37 sandblasting & spraying Cu 100prn 1 » E37 *, cure. Cold (thinner than Ell) 0.82 973 98 83 sandblasting & spraying Cu 100prn II LY5210 / HY2954, cured. hot, 120 ° C / 1 h 0.83 481 154 116 II LY5210 + 6% DT075 / HY2954 cure at 150 ° C / l h 0.70 1633 733 56 sandblasting & spraying Zn 100 pm sandblasting & spraying Zn 100 pm MY750 / HT972 cure hot, 150 ° C / 1h 0.73 1021 340 81 sandblasting & spraying Cu 100prn arc gun sandblasting & spraying Cu 100 pm arc gun 11 0.77 1383.5 190.5

E11 and E37 are different two-component epoxy adhesives (Permabond of Eastleigh, Hampshire, England) that can cure at room temperature and are somewhat denser. Experiments 26, 28, 24, 37 and 80 show the suitability of copper spraying on both surfaces in relation to resistance. The resistance obtained between the upper surface of the carbon and the aluminum is 1 W or less and includes a carbon resistance. This corresponds to a conductivity of the bond between the upper surface of the carbon and the metal of 190 Wm @ ² or greater. in cut. Note that the shear strength at ambient temperature may differ significantly from the shear strength at elevated temperature, e.g.

-5 CZ 282537 B6

160 ° C. Such temperatures are usually reached when using current collectors.

Experiment 79 shows that copper cladding, which forms a perfectly non-porous metal layer, does not provide the advantageous properties of copper spray that forms a porous metal layer.

The resin system experiment 83 includes Ciba resin LY5210 and HY2954 hardener mixed in a ratio of 100: 53 parts by weight. The shear strength was low because the resin mixture was very thin and soaked in carbon. This disadvantage was overcome in experiment 116 with a resin system that used the same resin mixture altered by adding 6 parts of silica powder (Ciba DT075) to 100 parts of resin base.

In Experiment 56, zinc was deposited in flame instead of copper, but resins and other factors were identical to Experiments 51 and 70.

In experiment 81, copper was deposited with an arc gun instead of a flame gun, but the resin and other factors were identical to experiments 51 and 70.

The resistance measurement is described below with reference to Fig. 2, in which the test component is held by an aluminum body 2 in a vice 5, which is connected to a negative DC power output. The positive DC power output is connected to a resistance probe consisting of a copper cylinder with three contact points at the bottom. The potential difference between the carbon surface and the conductive aluminum is measured using an insulated spring-pressed tip in the center of the copper cylinder, where the tip is connected to the voltmeter 6 by a second inlet directly connected to the aluminum body

2. The power supply is a constant current DC unit set at 5 A, which is accurately measured using a current shunt connected in series with a resistive clamping device.

In operation, the hand-held resistance probe is applied to the carbon surface at the measurement point and compressed to overcome the spring pressure of the stylus. The current is set to 5.0 A +/- 0.1. The mV measurement on the voltmeter 6 is recorded at certain points on the sample and the resistance is calculated according to Ohm's law.

The shear strength measurement is described below with reference to Fig. 3.

A 30 mm long section is cut from the pantograph strip 7. A hole 8 with a diameter of 2 mm, a depth of 15 mm and at a distance of 3 mm from the carbon-aluminum contact is drilled into the sample. A thermocouple is inserted into the opening 8 and the sample is secured to the shear test device by means of a pin 5. The shear force is exerted along the length of the pantograph.

The shear test apparatus consists of a heated pusher plate on the upper surface of which is a holder 1 which can slide under the hammer 12 and be locked in position by using a steel tip 13. The hammer position 12 relative to the specimen is critical and the steel tip 13 ensures stable placement sample to allow the hammer 12 to exert a force on the carbon near the contact with the aluminum. The entire test device is mounted on a Dennison Universal Force measuring instrument mounted and adjusted by means of a guide pin 14 located at the bottom of the shear test device.

The logger and temperature controller are turned on and the desired test temperature is set on the temperature controller. Temperature rise is monitored and controlled to ensure that the temperature is not exceeded. When the desired temperature is reached, the Dennison press is lowered and a force is applied to the hammer 12. Movement speed is 3.18 mm / min. The force is increased until the carbon is cut off from the aluminum and this maximum developed force is measured and recorded.

-6GB 282537 B6

The foregoing description relates to electric current collectors, but it is clear that the arrangement could be adapted to be used for brushes in motors, dynamos and alternators.

Claims (4)

PATENT CLAIMS A method for producing an electrically conductive joint between a carbon body having a carbon contact surface and a metal body having a metal contact surface, comprising the steps of forming a porous metal coating on at least a carbon interface surface and bonding the interface surfaces with an adhesive penetrating the adhesive pores of the metal coating and penetrating pores of the carbon contact surface. Method according to claim 1, characterized in that the metal contact surface is provided with a porous metal coating. Method according to claim 1 or 2, characterized in that the metal and carbon contact surfaces are sandblasted before the porous metal coating or coatings are applied. Method according to any one of claims 1 to 3, characterized in that hot-curing adhesives are used and the carbon body and the metal body are pressed together during curing of the adhesive by a pressing device which allows sliding along the joint during thermal expansion between carbon and metal.